1. Field of Invention
This application describes improvements to an ocular with an intermediate image plane at which possible locations at least one spatially modulating, reflective device is placed. The invention provides improvements to the scant examples of this type ocular. Different functions for the reflective device include nonlinear image light modulation; planar image processing; reticle, grid or text superposition, and these functions require sharp image focus on the device surface plane. Improvements relate to optical energy efficiency, sharpened focus and reduced keystoning while increasing the field-of-view for the observed virtual image of external scenery. Use with the eye, a camera or other instrument substituting for the eye is intended.
2. Description of Prior Art
There are many instances in which some modification of viewed scenery is desired. In the simple case of a telescopic gunsight, an intermediate image is formed, and an aiming reticle is placed at this image plane. The observer thus sees virtual imagery with the reticle clearly superimposed. In this case, the reticle may be affixed on glass or composed of cross-hairs neither of which obscures the viewed scenery. In other applications, the device in the intermediate plane is more complex than reticle cross-hairs and cannot transmit light through; being able, however, to modulate imagery through reflection. A case in point is the selective glare reduction ocular (SGRO) of Smith. See FIG. 1a for the ocular configuration. The purpose of that invention, conceived and witnessed in 1991 and issued at a patent in 1998 (U.S. Pat. No. 5,797,050) is to significantly reduce only the brightest parts of brilliant viewed scenery by using a nonlinear attenuator device (NAD) in the intermediate image plane. The invention enables the observer to view moderately illuminated objects in scenery and not be overcome by nearby bright objects in that scenery. Since there are light sensitivity advantages in having a reflective, rather than transmissive, NAD one of the ocular designs shows a reflective NAD at the intermediate plane.
The Smith patent specification teaches necessary concepts to enable one skilled in the art of photoconductor-liquid crystal devices to modify the ferroelectric liquid crystal devices described by Ivanova et. al., the Hamamatsu parallel aligned nematic spatial light modulator (PAL-SLM), and other modulators and use them for a reflective NAD. Component photoconductor films can include sensitized photogenerator-charge transport films of polymer, amorphous .alpha.-Si:H and pin .alpha.-SiCH, crystalline and polycrystalline films of CdS, GaP and ZnSe, and other materials. The common feature of all these options is the same as that for examples in the Smith patent: Optical energy on the layered device induces, via the photoconductor, change in the imposed electric field which causes liquid crystal reorientation, and, in conjunction with properly oriented polarizer(s), this translates into attenuation of reflected optical energy. This common feature is described in one of the patent's independent claims
Shortly after Smith's application to the aforementioned patent, Tomilin et. al. (1997) independently published a paper sketching a reflective NAD arrangement in an ocular for glare protection. A transmissive type NAD ocular was the main point of the paper, however, and it did not enumerate the complementary characteristics necessary for proper operation of a reflective NAD as did the claims in Smith's patent. Another example of prior art devices for which the ocular described in this application can be employed is the real-time reticle utilization of a liquid crystal light valve described by Beard et. al. (1973). The device modulates reflectivity on its front side according to a CRT image projected onto its rear side. Successors to this type of spatial light modulator is also used in projectors and has higher resolution than active matrix addressed liquid crystal displays, which generally modulate transmittance. Used as a reflective reticle in a sighting instrument, however, the light valve is able to change reticle patterns and impose symbology on imagery in real-time. Yet another example of prior art using a device in the intermediate image plane is the optical power limiter described by Liu (1990). This system is located in the "ocular" for an imaging sensor which needs protection from damaging intensity. If the ocular arrangement were reconfigured so that the power limiting layer were backed by a mirror and became a reflective power limiting device, it would be more sensitive (have lower threshold) because the optical energy passes through the same layer twice. Another invention in the category of power or intensity limiting which could be improved by a reflector behind the device in the intermediate image plane is that of Morse (1973) who used a scattering liquid crystal/photoconductor device. The cost of using a mirror to adapt the latter two inventions to that of an ocular with reflective device in the intermediate plane is a more complex design and acquiring the same problems that occur with all oculars which are the subject of this invention. These problems are discussed below.
The germane point of the foregoing paragraphs on prior art is to establish the existence of (and need for) ocular designs in which a spatially modulating reflective device is placed at the intermediate image plane. Whether the reflective device superimposes text on a virtual image, modulates the contrast, serves as an optical power limiter or serves a multiple purpose is of little consequence to the ocular design improvements needed and described in this application. Therefore problems and solutions connected with this type ocular have very wide application relevance.
3. Problems
The problem with an ocular having a spatially modulating, reflective device in the intermediate plane is the difficulty of getting uniform focus of the image on that device. (See FIG. 1a which depicts the ocular arrangement of one embodiment of Smith's SGRO patent of 1998). Another aspect of this problem is the difficulty of viewing features with uniform focus which the reflective modulator device 25 may impose on the imagery. This problem arises because the most light-efficient use of the reflective device has rays of optical energy 10 coming to and leaving the reflective device plane at widely different angles. This means the intermediate image focal surface 35 is not coincident with the plane of the reflective device. Near-focus can occur only over a small area which translates to a very narrow field-of-view in the virtual imagery. An associated problem is the keystoning. The field of the image projected onto the plane of the reflective device is distorted into a "keystone" shape.
One solution to non-focus and keystoning problems at the reflective device, which is familiar to experienced photographers, is to cant the objective and eyepiece lenses as shown in FIG. 1b so that their curved focal surfaces 35 and 40 essentially coincide with the device plane. Then a field lens 30 is placed in proximity to the surface of the reflective device which does not change focus of any imagery appreciably but increases the field-of-view seen through eyepiece 47. Such an increase is justified due to the larger region of improved focus and decreased field distortion.
For applications with lenses having low f/stop-number, this solution has a drawback because the canted lenses 17 and 47 produce too much off-axis aberration (especially coma) at the surface plane of the reflective device 25. An alternative solution is to use certain beamsplitters (e.g., transparent plates, thin metal film-coated plates or cube beamsplitters composed of two right-angle prism halves joined by metal films and cement) which can get optical energy to the device at near-normal incidence so that a subsequent, wider field-of-view accommodation is justified. The simple beamsplitter approach has been applied to entirely different applications and described, for example, in literature by Fein (1970), Smith (1979), Shionoya (1984) and Rosenbluth (1990). This alternative, however, results in an ultimate optical energy throughput of less than 25%, at best. Such a situation is undesirable and requires an additional component for improvement.